Hydraulic machines such as excavators, dozers, loaders, backhoes, motor graders, and other types of heavy equipment use one or more hydraulic actuators to accomplish a variety of tasks. These actuators are fluidly connected to an engine-driven pump of the machine that provides pressurized fluid to chambers within the actuators. As the pressurized fluid moves into or through the chambers, the pressure of the fluid acts on hydraulic surfaces of the chambers to affect movement of the actuators and a connected work tool.
Swing-type excavation machines, for example hydraulic excavators and front shovels, require significant hydraulic pressure and flow to transfer material from a dig location to a dump location. These machines direct the high-pressure fluid from an engine-driven pump through a swing motor to accelerate a loaded work tool at the start of each swing, and then restrict the flow of fluid exiting the motor at the end of each swing to slow and stop the work tool.
One problem associated with this type of hydraulic arrangement involves efficiency. In particular, the pressurized oil provided by the pump may slowly accelerate the work tool to its steady state swing speed, making the hydraulic system less responsive to the operator swing commands than is desirable to efficiently complete the required tasks. Moreover, the fluid exiting the swing motor at the end of each swing is under a relatively high pressure due to deceleration of the loaded work tool. Unless recovered, energy associated with the high-pressure fluid may be wasted. In addition, restriction of this high-pressure fluid exiting the swing motor at the end of each swing can result in heating of the fluid, which must be accommodated with an increased cooling capacity of the machine.
One attempt to recover swing kinetic energy in a swing-type machine is disclosed in U.S. Pat. No. 8,850,806 to Zhang et al. issued on Oct. 7, 2014 (the '806 patent). The '806 patent discloses a hydraulic control system for a machine that may have a work tool movable through segments of an excavation cycle, a motor configured to swing the work tool during the excavation cycle, at least one accumulator configured to selectively receive fluid discharged from the motor and to discharge fluid to the motor during the excavation cycle, and a controller. The controller may be configured to receive input regarding a current excavation cycle of the work tool, and make a determination based on the input that the current excavation cycle is associated with one of a set of known modes of operation. The controller may be further configured to cause the at least one accumulator to receive fluid by actuating an electro-hydraulic charging valve, and to discharge fluid by actuating an electro-hydraulic discharge valve, during different segments of the excavation cycle based on the determination. The arrangement with two electro-hydraulic valves provides flexibility in design as the performance of the valves is tuned to the particular machine in which the hydraulic control system is implemented. The discharge valve discharges recovered energy in the accumulator directly back to the swing motor during swing acceleration. However, swing acceleration performance can vary based on the pressure within the accumulator at a given time. Moreover, the discharge valve cannot be opened during charging portions of the excavation cycle, so excess kinetic energy may be wasted or lost once the accumulator is fully charged. Therefore, opportunities exist for providing energy recovery systems in swing-type machines that provide more consistent performance in swing acceleration, are more portable between different sizes and types of machines, and are more efficient at capturing kinetic energy.
Another efficiency issue associated with these types of hydraulic arrangements arises during times when the hydraulic machine is idle and yet still operational. For example, during a truck loading cycle, when an excavator finishes loading a first truck, the excavator must wait for the first truck to depart and a second truck to arrive before additional loading tasks can be completed. During this time, the engine of the machine may still be turned on (often at high speeds) and needlessly consuming fuel. In these situations, it may be beneficial to selectively turn the engine off to consume fuel. However, restarting the engine can be harsh on the machine's electrical circuit and cause delays in the work cycle of the machine. Specifically, the electrical circuit could be called on to restart the engine hundreds of times during a particular work shift. In some applications, this overuse of the electrical circuit could cause premature wear and/or failure. In addition, it may take some time for the engine to be turned on and ramp up to required speeds. This time delay could result in loss of productivity and/or become a nuisance for the operator.
An engine-assist device and industrial machine is disclosed in Int'l. Publ. No. 2014/115645 A1 to Shigeo et al. published on Jul. 31, 2014 (the '645 publication). The '645 publication discloses a low-cost engine-assist device that can perform stable energy regeneration from an accumulator, and an industrial machine equipped with the engine-assist device. Variable-capacity main pumps and a variable-capacity assist pump having a motor function and a pump function are directly coupled to an engine. Return pressure fluid that has flowed out of fluid pressure actuators is temporarily stored by a sub-accumulator and supplied to an inlet of the assist pump, and the assist pump provides increased pressure to the main accumulators. A controller calculates and controls an assist pump swash plate angle by means of engine load torque, or of assist starting torque or charge starting torque set by an engine speed setting means, and the controller conducts the stored pressure fluid discharged from the main accumulators to the inlet of the assist pump or conducts the increased-pressure fluid discharged from the outlet of the assist pump to the main accumulators.
Current engine anti-idling systems such as that disclosed in the '645 publication have their own charge/discharge system. The separate system shares the accumulator with the swing circuit to store the energy required to restart the engine, and uses the assist motor with the boom circuit to restart the engine. The accumulator is charged if necessary before shutting down the engine by using main pump flow. Such systems may not be highly efficient because the main pump works at high pressure and low flow rate, and hardware redundancy occurs where separate accumulate charge and discharge valves are provided for the anti-idling system. Therefore, opportunities exist for improving the efficiency and integration of anti-idling systems with the swing and boom circuits.